1. Field of the Invention
The present invention relates to a positive electrode active material and non-aqueous secondary battery using the same capable of increasing a molding density (packing density) of the active material in a positive electrode, and capable of improving discharging rate characteristic of the battery by lowering an impedance of the electrode.
2. Description of the Related Art
In recent years, developments of a relatively safe negative electrode material and a non-aqueous electrolyte having an increased decomposition voltage have been advanced, so that various non-aqueous secondary batteries having a high operating voltage have been practically used in many technical fields. In particular, a secondary battery using a lithium ion has excellent characteristics such as a high discharge voltage, light weight, and a high energy density or the like, so that the demand of the secondary battery has been rapidly increased as power sources of equipments and devices such as portable telephone (cellular phone), notebook-sized personal computer, camera-integrated video recorder, and as dispersed-type power sources, power sources of EV (electrical vehicle) and HEV (hybrid electrical vehicle), and a large-scaled battery.
The lithium ion secondary battery of this type comprises: a positive electrode containing the active material and a negative electrode containing carbon, that are capable of reversibly deintercalating/intercalating lithium ion; and a non-aqueous electrolyte which is prepared by dissolving lithium salt into non-aqueous solvent.
As the positive electrode active material for the above lithium ion secondary battery, for example, lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMn2O4 and other metal oxides have been generally used.
As the positive electrode for the above secondary battery, there has been generally used a positive electrode formed in such a manner that a mixture of positive electrode material consisting of Li-containing transition metal composite oxide, conductive material and binder is coated onto an Al plate as a collector, then the coated mixture is dried and followed by press-compacting the dried material.
In order to increase a charging/discharging capacity per unit volume of the above secondary battery, it is required to increase a finished density (final density) of a positive electrode material layer containing the above active material, filler, binder or the like. Therefore, for the purpose of increasing the density of the active material layer, there has been adopted a method comprising steps of: forming a positive electrode active material layer (positive electrode film); and thereafter, conducting a press-rolling operation for densifying the positive electrode film thereby to increase the density thereof.
Even if the conventional positive electrode active materials were press-densified by the press-rolling operation, the pressed material could attain a density to some extent, however, the resulting density was still insufficient indeed. That is, even if the pressing pressure is increased so as to obtain a high density for the positive electrode active material layer, a high density cannot be obtained no longer when the pressing pressure exceeds predetermined pressure level, so that there has been raised a problem that a sufficient charging/discharging capacity for the secondary battery cannot be obtained.
Further, although only a surface portion of the active material layer can be highly densified, a portion apart from the surface and close to the collector cannot obtain a sufficiently high density. At any rate, there has been raised a problem that a charging/discharging capacity and discharging rate characteristic of the secondary battery become insufficient.
Furthermore, there has been also proposed a secondary battery using a positive electrode formed in accordance with alkoxide method in which a thin film-shaped (layer-shaped) oxide layer is formed on the surface of the positive electrode active material. However, in the active material having such thin film, migration and movement of lithium ion are obstructed, so that there is posed a problem of disadvantageously lowering the battery characteristics.
On the other hand, a notebook-sized personal computer, a personal digital assistant (PDA), a cellular phone or the like have been rapidly come into wide use in recent years, and a realization of a mobile computing system is remarkably progressed. To cope with the situation, multi-functioned portable electronic devices or the like have been required to be operated for a long time. Therefore, with respect to the secondary battery to be used as the power sources for various equipments including the portable electronic devices, a technical demand for realizing compact size and a high capacity for the batteries has been raised.
The present invention had been achieved to solve the aforementioned problems and an object of the present invention is to provide a positive electrode active material and a non-aqueous secondary battery capable of increasing molding density (packing density) of the active material in a positive electrode, and capable of improving discharging rate characteristic of the battery by lowering an impedance of the electrode.
In order to achieve the aforementioned object, the inventors of this invention had prepared positive electrode active materials by adhering particles as various adhered substances onto active materials having various compositions, and the inventors had comparatively reviewed the influences of kind, adhering amount, grain size of the particles on the densifying property of the active materials.
As a result, the inventors had obtained the following knowledge. Namely, when fine oxide particles or carbon particles were adhered onto the surface of the active material body thereby to prepare a positive electrode active material, the packing characteristic of the active material could be significantly improved.
More concretely to say, the following findings were obtained. When particles of oxide of specified elements such as Bi, Sb, Al, Sn or the like were adhered to the surface of the active material body thereby to prepare the positive electrode active material, friction force among the active materials was greatly reduced thereby to increase a flowability of the active material, so that a positive electrode film having a higher density than that of the conventional one could be obtained through a rolling operation under a high pressure. As a result, there could be obtained a secondary battery excellent in charging/discharging characteristic and capacity.
In addition, when carbon particles such as general carbon black, electrically conductive carbon black, acetylene black or the like were adhered to the surface of the active material body consisting of Li-containing transition metal composite oxide thereby to prepare the positive electrode active material, electrical conductivity among the positive electrode active materials was greatly improved, so that it became possible to reduce an amount of filler which had been used in the conventional positive electrode active material layer (film) for the purpose of increasing the electrical conductivity, and thus also making it possible to increase density of the positive electrode active material layer and to improve the discharging capacity.
Furthermore, in also a case where the carbon particles were adhered to the surface of the active material, friction force among the active materials was greatly reduced thereby to increase the flowability of the active material, so that a positive electrode film having a higher density than that of the conventional one could be obtained through a rolling operation under a high pressure. As a result, there could be obtained a secondary battery excellent in charging/discharging characteristic and capacity.
In addition, the following finding was also obtained. Namely, when at least one of the oxide particles and the carbon particles were adhered to the active material and the active material was subjected to the press-rolling operation as described above, there could be obtained a positive electrode film having a uniform density of the active material in a depth direction of the positive electrode, so that an impedance of the positive electrode was lowered whereby a low-temperature characteristic and discharging rate characteristic of the secondary battery could be improved. The present invention had achieved on the basis of the aforementioned findings.
That is, a positive electrode active material according to the present invention is characterized by comprising: a positive electrode active material body; and at least one of oxide particles and carbon particles each having an average diameter of 1 xcexcm or less; wherein at least one of oxide particles and carbon particles are adhered to a surface of the positive electrode active material body.
In the above positive electrode active material, it is preferable that the oxide particles adhered to the positive electrode active material body are particles composed of oxide of at least one element selected from the group consisting of Si, Sn, Al, Ti, Mg, Fe, Bi, Sb and Zr.
Further, it is also preferable that the oxide particles adhered to the positive electrode active material body are composed of at least one oxide particle selected from the group consisting of SiO2, SnO2, Al2O3, TiO2, MgO, Fe2 O3, Bi2O3, Sb2O3 and ZrO2.
Furthermore, it is also preferable that the mass of the oxide particles adhered to the positive electrode material body is 0.001-2% of a mass of the positive electrode active material body.
Further, it is also preferable that the mass of the carbon particles adhered to the positive electrode material body is 0.001-10% of a mass of the positive electrode active material body.
Although a composition of the active material to be used as the body of the positive electrode active material according to the present invention is not particularly limited, it is preferable that the positive electrode material body is a transition metal composite oxide containing lithium expressed by a general formula:
LixMyOzxe2x80x83xe2x80x83(1) 
wherein M denotes at least one element selected from transition metals, and 0.8xe2x89xa6xxe2x89xa61.15, 0.8xe2x89xa6yxe2x89xa62.2 and 1.5xe2x89xa6zxe2x89xa65.
In particular, it is more preferable to use a transition metal composite oxide containing lithium expressed by a general formula:
LixMyO2xe2x80x83xe2x80x83(2) 
wherein M denotes at least one element selected from transition metals, and 0.8xe2x89xa6xxe2x89xa61.15 and 0.8xe2x89xa6yxe2x89xa61.1.
In addition, it is also preferable to use a transition metal composite oxide containing lithium expressed by a general formula:
LixMyO4xe2x80x83xe2x80x83(3) 
wherein M denotes at least one element selected from transition metals, and 0.8xe2x89xa6xxe2x89xa61.1 and 1.8xe2x89xa6yxe2x89xa62.2.
In the Li-containing transition metal composite oxide expressed by the above general formulas (1) to (3), various transition metals such as Co, Ni, Mn, Fe, V or the like can be used as M element. In particular, it is effective to use Co as at least part of M element.
Further, it is also effective to use at least one element selected from Co and Ni as M element. It can be said that the above Li-containing Co composite oxide is preferable in view of a battery capacity. Furthermore, it is also preferable that a part of M element is replaced with metal element other than transition metal.
In the general formulas (1) and (2), a value of x is set to a range of 0.8-1.15, a value of y is set to a range of 0.85-1.1, and a value of z is set to a range of 1.5-5 respectively. On the other hand, In the general formula (3), a value of x is set to a range of 0.8-1.1 and a value of y is set to a range of 1.8-2.2 respectively. In any cases where the values of x, y and z are out of the above ranges, a sufficient battery capacity cannot be obtained. It is preferable that a ratio of x/y is set to 1 or more. When the ratio satisfies a relation x/y less than 1, a sufficient crystallizing property cannot be obtained thereby to lower a cycle characteristic and battery capacity.
In addition, the oxide particles composed of oxide of element selected from the group consisting of Si, Sn, Al, Ti, Mg, Fe, Bi, Sb, Zr or the like, preferably oxide particles or composite oxide particles selected from the group consisting of SiO2, SnO2, Al2O3, TiO2, MgO, Fe2O3, Bi2O3, Sb2O3 and ZrO2 to be adhered to the surface of the positive electrode active material body have a function of decreasing the friction force caused among the positive electrode active material particles. Therefore, the particles are used to increase the flowability of the active material and used to form a positive electrode active material layer having a high density when the positive electrode active material is press-molded to the collector.
Further, as the carbon particles to be adhered to the surface of the positive electrode active material, electrically conductive carbon black, acetylene black, hydrophilic carbon black, graphite can be preferably used in addition to the general carbon black. In particular, hydrophilic carbon black, which is formed by subjecting a surface reforming treatment to hydrophobic carbon black, is greatly improved in dispersibility into a water solution. Therefore, the hydrophilic carbon black is particularly effective in a case where water is used as dispersion medium.
The above carbon particles have a function of improving the electrical conductivity among the active material particles and a function of reducing the friction force caused between the positive electrode material particles, so that the carbon particles are used to increase the flowability of the active material and used to form a positive electrode active material layer having a high density when the positive electrode active material is press-molded to the collector.
In particularly, when an average grain size of the oxide particles or carbon particles to be adhered to the surface of the positive electrode active material body is set to 1 micron or less, the effect of lowering the friction force and the effect of densifying the active material layer can be further increased. It is further preferable that the average grain size of the oxide particles or carbon particles to be adhered to the surface of the positive electrode active material body is set to 0.1 micron or less.
In this connection, when coarse oxide particles or carbon particles of which average grain size (preliminary grain size) exceeds 1 xcexcm is used, in order to obtain the above effects, a large amount of oxide particles or carbon particles are required to be adhered to the active material body. In this case, however, the packing density of the Li-containing transition metal composite oxide acting as a main substance of battery reaction is relatively lowered, so that a positive electrode exhibiting high battery characteristics cannot be obtained.
In the present invention, the above average grain size of the oxide particles or carbon particles is measured in accordance with the following manner. That is, a surface of an arbitral active material is observed by means of a scanning-type electron microscope (SEM), then substances on the surface are identified by EPMA (Electron Probe Micro Analyzer) there by to prepare a SEM image, and the sizes of the identified 10 pieces of oxide particles or carbon particles are measured from the SEM image. The average grain size is given by averaging the measured sizes.
In addition, it is preferable that the mass of the oxide particles adhered to the positive electrode material body is 0.001-2% of a mass of the positive electrode active material body. When the amount of the adhered particles is less 0.001%, the effect of lowering the friction force and the effect of press-densifying the active material layer cannot be obtained. On the other hand, when the adhesion amount of the particles is excessively large so as to exceed 2%, the amount of the active material is relatively decreased thereby to lower the battery characteristics. Therefore, the adhesion amount is set to the above range. However, the range of 0.005-1% is more preferable, and the range of 0.006-0.5% is further more preferable.
Further, it is preferable that the mass of the carbon particles adhered to the positive electrode material body is 0.001-10% of a mass of the positive electrode active material body. When the amount of the adhered particles is less than 0.001%, the aforementioned effect of improving the electrical conductivity, the effect of lowering the friction force and the effect of press-densifying the active material layer cannot be obtained. On the other hand, when the adhesion amount of the particles is excessively large so as to exceed 10%, the amount of the active material is relatively decreased thereby to lower the battery characteristics. Therefore, the adhesion amount is set to the above range. However, the range of 0.01-8% is more preferable, and the range of 0.01-3% is further more preferable.
Furthermore, a method of manufacturing the aforementioned active material body is not particularly limited. However, the positive electrode active material having above characteristics can be manufactured with a high production yield in accordance with, for example, the method comprising the steps of: blending cobalt compound and lithium compound, or at least one compound selected from the group consisting of cobalt compound, manganese compound, iron compound and aluminum compound, with a mixture consisting of cobalt compound and lithium compound so that an atomic ratio Li/(Ni+M), i.e. the atomic ratio of lithium with respect to total amount of cobalt and the replaced element M, is set to a range of 1.0-1.2 in terms of molar ratio at blending step thereby to prepare a mixed material; and retaining the mixed material at a temperature range of 680-1100xc2x0 C. in air-flowing atmosphere thereby to perform a heat treatment.
As the above molar ratio, a range of 1.02-1.15 is more preferable. Further, a more preferable temperature range for the heat treatment is a range of 800-1000xc2x0 C. In this connection, in the heat treatment step, the heat treating operation may be also performed in such a manner that the mixed material is stepwisely heated up through at least two stage-heating within a temperature range of 400-950xc2x0 C.
As the above cobalt compound, for example, cobalt oxide, cobalt carbonate, cobalt nitrate, cobalt hydroxide, cobalt sulfate, cobalt chloride or the like can be preferably used.
Further, as the above lithium compound, for example, lithium hydroxide, lithium oxide, lithium carbonate, lithium nitrate, lithium halogenide or the like can be preferably used.
Furthermore, as the above nickel compound, for example, nickel hydroxide, nickel carbonate, nickel nitrate or the like can be preferably used.
Still further, as the above manganese compound, for example, manganese dioxides such as electrolytic manganese dioxide (EMD) or chemically prepared manganese dioxide (CMD), manganese oxyhydroxide, manganese carbonate, manganese nitrate or the like can be preferably used.
Furthermore, as the above iron compound, for example, iron oxide, iron hydroxide, iron carbonate, iron nitrate or the like can be preferably used.
Furthermore, as the above aluminum compound, for example, aluminum oxide, alumina hydrate, aluminum hydroxide, aluminum nitrate or the like can be preferably used.
Further, aforementioned heat treating step can be performed by using an electric furnace equipped with an atmosphere intake mechanism or a continuous-type furnace such as open-type pusher furnace. Further, it is preferable to perform aforementioned heat treatment for 1 to 20 hours. In this connection, a gas to be introduced into the furnace is not limited to air. When an oxygen flow is introduced into the furnace as occasion demands, it becomes also possible to increase a production efficiency of the composite oxide active material.
In addition, after the positive electrode active material body is synthesized as described above, the operation of adhering at least one particle selected from oxide particles and carbon particles onto the surface of the active material body can be performed in accordance with the following procedures. That is, thus prepared active material body is dispersed into water thereby to prepare a dispersion, while a dispersion of the oxide/carbon particles having a fine average grain size is prepared. Thereafter, a predetermined amount of the oxide or carbon particle dispersion is introduced into the active material body dispersion, and the dispersion is uniformly stirred. When the stirred dispersion is thickened and dried whereby there can be obtained a positive electrode active material in which the particles are integrally adhered to the surface of the active material body.
In this connection, for the purpose of realizing a uniform dispersion property, it is preferable to prepare the dispersion of the active material body particles and the dispersion of the oxide/carbon particles respectively, and then mix the two dispersions. However, it is also possible to mix the active material body particles with the oxide/carbon particles in a dry process without preparing the dispersion of at least one of the active material body particles and the oxide/carbon particles.
Further, after the positive electrode active material body is synthesized as described above, the operation of adhering the carbon particles onto the surface of the positive electrode active material body can be also performed in accordance with the following procedures. That is, there can be also adopted a method comprising: wet-type dispersing process in which the positive electrode active material particles and the carbon particles are dispersed into a dispersion medium consisting of water or an organic solvent thereby to prepare a dispersion; and dry process in which the dispersion medium is evaporated from thus obtained dispersion thereby to adhere the carbon particles onto the surfaced of the positive electrode active material particles.
In the above treating method, it is preferable that the above drying process is performed in accordance with a drying method selected from fluidization drying method, spray drying method, suction drying method and heat drying method.
A non-aqueous secondary battery according to the present invention is constituted by comprising: a positive electrode in which the positive electrode active material as prepared above, conduction filler (electric conductive agent) together with binder or the like are mixed and pressingly molded to be retained by the positive electrode; a negative electrode having a negative electrode active material; wherein the positive electrode and the negative electrode are provided through a non-aqueous electrolyte in a battery case so that the positive and negative electrodes are opposed to each other.
In this regard, as the conduction promoting agent described above, for example, acetylene black, carbon black, graphite or the like are used. Further, as the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-butadiene-diene interpolymer (EPDM), styrene-butadiene rubber (SBR) or the like can be available.
Further, the positive electrode described above is manufactured, for example, in such a manner that the positive electrode active material described above and the binder are suspended in an appropriate solvent to prepare a suspended solution, then the suspended solution is coated onto a collector and dried, thereafter the coated collector is pressed by a pressing machine. In this regard, as a material for constituting the collector, for example, aluminum foil, stainless steel foil, nickel foil or the like are preferably used.
On the other hand, as a negative electrode active material, a material containing carbon material for deintercalating/intercalating lithium ion, or a material containing chalcogen compound, or an active material comprising a light metal can be used. In particular, when the negative electrode containing carbon material or chalcogen compound is used, the battery characteristics such as cycle life or the like is improved, thus being particularly preferable.
As examples of the carbon material for absorbing/releasing lithium ion, for example, coke, carbon fiber, pyrolytic carbon material, graphite, resin sintered body, mesophase-pitch-type carbon fiber (MCF) or sintered bodies of mesophase spherical carbon or the like are used. In particular, when the liquid crystalline mesophase-pitch-type carbon fiber (MCF) prepared by graphitizing a heavy gravity oil at a temperature higher than 2500xc2x0 C. or mesophase spherical carbon is used, an electrode capacity of the battery can be increased.
Further, the above carbon materials preferably have an exothermic peak (heat peak) at a temperature of 700xc2x0 C. or more, more preferably, at a temperature of 800xc2x0 C. or more when the materials are subjected to a differential thermal analysis. Furthermore, assuming that (101) diffraction peak of a graphite structure of the carbon material detected by an X-ray diffraction (XRD) is (P101) and (100) diffraction peak is (P100), it is preferable that the carbon material has an intensity ratio of P101/P100 ranging from 0.7 to 2.2. In case of the negative electrode containing the carbon material having such the intensity ratio of the diffraction peaks, a rapidly absorbing or releasing the lithium ion can be performed, so that it is effective to combine the negative electrode with the positive electrode containing the aforementioned positive electrode active material which directs to the rapidly-charging/discharging operation.
As examples of the chalcogen compounds for absorbing/releasing lithium ion, for example, titanium disulfide (TiS2), molybdenum disulfide (MoS2), niobium selenide (NbSe2) or the like can be used. When the chalcogen compounds are used for the negative electrode, a voltage of the secondary battery is lowered, while the capacity of the negative electrode is increased, thereby to increase the capacity of the secondary battery. In addition, a diffusing speed of the lithium ion at the negative electrode is increased, so that it is particularly effective to combine the chalcogen compound with the positive electrode active material used in the present invention.
Further, as examples of the light metals to be used for the negative electrode, for example, aluminum, aluminum alloy, magnesium alloy, lithium metal, lithium alloy or the like can be used.
Furthermore, the negative electrode containing the active material for absorbing/releasing the lithium ion can be manufactured, for example, by a method comprising the steps of suspending the above negative electrode active material and the binder into inappropriate solvent to form a suspended solution, coating the suspended solution onto a collector, and drying then press-contacting the collector. As an example of the collector, for example, collectors composed of copper foil, stainless steel foil, nickel foil or the like are used.
Further, as the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-butadiene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or the like can be available.
Further, the separator described above is formed, for example, from nonwoven fabric composed of synthetic resin, polyethylene porous film, polypropylene porous film or the like.
As the non-aqueous electrolyte, a solution prepared by dissolving electrolyte (lithium salt) into anon-aqueous solvent is used.
The examples of the non-aqueous electrolyte may include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) or the like; chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC) or the like; chain ethers such as dimethoxyethane (DME), diethoxy ethane (DEE), ethoxymethoxy ethane or the like; cyclic ethers such as tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF) or the like; fatty acid ethers of lactone, xcex3-butyrolactone (xcex3-BL) or the like; nitrides such as acetonitrile (AN) or the like; and sulfides such as sulfolane (SL), dimethyl sulfoxide (DMSO) or the like.
The above non-aqueous solvent can be singularly used, and can be also used as a mixed solvent prepared by mixing at least two kinds of the above solvents. In particular, it is preferable to use; a solvent composed of at least one solvent selected from the group consisting of EC, PC and xcex3-BL; or a mixed solvent which is prepared by mixing at least one solvent selected from the group consisting of EC, PC and xcex3-BL with at least one solvent selected from the group consisting of DMC, MEC, DEC, DME, DEE, THF, 2-MeTHF and AN.
Further, in a case where the negative electrode active material containing carbon material for absorbing/releasing the lithium ion is used as a material for constituting the negative electrode, in view of improving the cycle life of the secondary battery comprising the negative electrode, it is preferable to use: a mixed solvent composed of EC, PC, and xcex3-BL; a mixed solvent composed of EC, PC, and MEC; a mixed solvent composed of EC, PC, and DEC; a mixed solvent composed of EC, PC, and DEE; a mixed solvent composed of EC and AN; a mixed solvent composed of EC and MEC; a mixed solvent composed of PC and DEC; a mixed solvent composed of PC and DEC; or a mixed solvent composed of EC and DEC.
The examples of the electrolytes may include lithium salts such as lithium perchlorate (LiClO4), lithium phosphate hexafluoride (LiPF6), lithium boride fluoride (LiBF4), arsenic lithium hexafluoride (LiAsF6), trifluoromethasulfonic lithium (LiCF3SO3), bis-trifluoromethyl sulfonylimide lithium [LiN(CF3SO2)2] or the like. In particular, when LiPF6, LiBF4, LiN (CF3SO2)2 are used, electric conductivity and safety can be improved, thus being preferable. The electrolyte containing LiBF4 has a function of protecting the surface of the positive electrode active material, thus being particularly preferable.
An amount of these electrolytes to be dissolved into the non-aqueous solvent is preferably set to a range of 0.1-3.0 mol/l. This is because, when the amount (concentration) of the electrolyte exceeds 3.0 mol/l so as to provide a high lithium salt concentration, a reaction between the positive electrode active material and the electrolyte becomes active at high temperature region, thus departing from the object of the present invention.
According to the positive electrode active material and the non-aqueous secondary battery using the active material as constructed above, since at least one of the oxide particles and the carbon particles were adhered onto the surface of the active material body thereby to prepare a positive electrode active material, the friction force among the active materials is greatly reduced thereby to increase a flowability of the active material, so that a positive electrode film having a higher density than that of the conventional one can be obtained through a rolling operation under a high pressure. As a result, it becomes possible to realize a secondary battery excellent in charging/discharging characteristic and capacity.
Particularly in a case where the carbon particles are adhered to the active material body thereby to prepare the positive electrode active material, electrical conductive paths between the active material are sufficiently secured and the impedance of the electrode film is advantageously lowered, so that the discharging rate characteristic can be significantly improved.